Of the noxious substances (such as hydrocarbons, carbon monoxide, and nitrogen oxides) contained in exhaust gases from an internal combustion engine, nitrogen oxides are said to be formed by the reaction of nitrogen (N.sub.2) with oxygen (O.sub.2), which are present in the intake air, at high termperature. Therefore, the presence of nitrogen oxides in the exhaust gas can be reduced by avoiding combusting the air-fuel mixture at high temperature. This avoidance of high-temperature combustion is attained by increasing the ratio of inert gas in the intake air. For this purpose, an exhaust gas recirculating system (hereinafter abbreviated the EGR system) which supplies the exhaust gas into the intake air is employed.
However, EGR is likely to lead to unstable combustion in the cylinders and consequently to misfire. Accordingly, when to permit EGR and when to cut if off becomes an important problem. Generally speaking, ignitability is low during idling, even without EGR. If EGR were permitted, ignitability would be further impaired, and a large quantity of hydrocarbons would be emitted, which is undesirable from the viewpoint of emission control. Thus, EGR is usually cut off during this period. EGR is also cut off during full-load operation, because this period necessitates generation of maximum output, and does not last very long. Further, during deceleration, when ignitability is lower than during idling, EGR is usually cut off to prevent further worsening of ignitability. It has also been proposed to cut off EGR during starting and when operating the engine at low temperature.
In order to perform the above-described function, the conventional EGR system has included an EGR control valve, of diaphragm type, provided in the EGR passage interconnecting the intake and exhaust passages, which valve is operated by a negative pressure. The negative pressure working on a vacuum port in the intake passage is introduced into the actuation chamber of the EGR control valve, and said valve is actuated when the negative pressure exceeds a predetermined level so as to mix the recirculated exhaust gas into intake air. Said vacuum port is disposed so as to be located upstream and downstream of the edge of a throttle valve provided in the intake passage when it closes and opens, respectively. This vacuum port is usually provided in the vicinity of the edge of the throttle valve so that it becomes located upstream of said edge when the throttle valve is in the idling position, and downstream thereof when the throttle valve moves out of the idling position. Because of this construction, the conventional EGR system cuts off EGR only when the throttle valve is in the idling position, i.e., during the idling and deceleration of the engine, and when the throttle valve is substantially fully opened to make the pressure working on the vacuum port substantially atmospheric, i.e., during the full-load operation of the engine.
However, the recent tendency is to increase the quantity of the recirculated exhaust gas introduced into the intake so as to reduce the nitrogen oxides more effectively. In so doing, however, the temperature of the recirculated exhaust gas exceeds the heat-resisting limit of the EGR control valve in the high-speed, high-load operating condition, which is likely to give rise to burning and/or sticking of said valve and other troubles. It therefore becomes necessary to also cut off EGR in such an operating condition.
The primary object of this invention is to provide an EGR system that cuts off EGR not only during the idling, deceleration and full-load operations, but also in the partially laoded region if the quantity of air sucked is large, and, further, proportions the quantity of the recirculated exhaust gas to that of sucked intake air.
The feature of this invention lies in an EGR system that comprises, in combination, a constricted portion provided in an EGR passage interconnecting the intake and exhaust passages of an internal combustion engine, an EGR control valve provided downstream thereof, a negative control pressure passage introducing the negative control pressure from a vacuum port in the intake passage to said EGR control valve, a constant-pressure chamber located between said constricted portion and said EGR control valve so that the pressure inside lowers to atmospheric when the EGR control valve opens, since said chamber communicates with the intake passage, and becomes equal to the pressure in the exhaust passage when said valve closes, a first operating valve opening said negative control pressure passage to the atmosphere when the pressure in said constant-pressure chamber becomes substantially atmospheric, and a second operating valve opening said negative control pressure passage to the atmosphere when the pressure in the intake passage, which indicates the amount of intake air flowing therethrough, exceeds a predetermined negative pressure level, and further including means for detecting such factors as water temperature, drive speed and gear shifting for indicating the operating condition of an engine, and opening said negative control pressure passage to the atmosphere when the detected operating condition reaches a predetermined condition, when necessary.